1. Field
The present application relates to garnet oxide phosphor compositions co-doped with Ce and Mn, as well as methods of making the same. The present application also relates to lighting devices including said composition.
2. Description
Solid state light emitting devices, such as light emitting diode (LED), organic light emitting diode (OLED) or sometimes called organic electroluminescent device (OEL), and inorganic electroluminescent device (IEL), are widely utilized for various applications such as flat panel displays, indicators for various instruments, signboards, and ornamental illumination, etc. Improving the efficiency of these light emitting devices, especially LED, may provide higher luminescence intensities comparable to general purpose lighting devices, such as fluorescent and incandescent lamps. A white-LED, especially with a high color rendering index (CRI) and low correlated color temperature (CCT), shows promise as a replacement for existing general purpose lighting devices.
Conventionally, white-LED includes the combination of blue-LED and yellow light emitting YAG phosphor powder dispersed in an encapsulating resin, such as an epoxy or silicone (see e.g, U.S. Pat. Nos. 5,998,925 and 6,069,440). However, this YAG:Ce type LED system exhibits low CRI due to the lack of red luminescence. Accordingly, there is a need for phosphors providing broad spectral emission over a wider range of wavelengths, as well as simple processes for preparing these phosphors.
One approach for obtaining a broad emission spectrum has been to use Ce as an activator to provide a wide emission spectrum with peak positions from the green (about 480 nm to about 580 nm) to orange regions (about 585 nm to about 620 nm). Because the emission of Ce ions is associated with electron transitions from 5d orbitals, the emission spectra can be shifted by other cations present in the host lattice. It is reported that Lu2CaAl4SiO12:Ce exhibits green emission when excited by blue LED (U.S. Pat. No. 7,029,602). Thus, Lu2CaAl4SiO12:Ce exhibits a blue-shifted emission relative to Y3Al5O12:Ce, which results from the smaller ionic size of Lu.
Mn ions usually exhibit emission in the green or red region depending upon the crystalline structure and lattice position. As such, Mn ions could theoretically improve the emission spectra of existing phosphors. Nevertheless, Mn ions poorly absorb blue light, and therefore only exhibit luminescence when excited by UV radiation. Consequently, Mn is not typically used in conventional white-LEDs having blue-LEDs to excite the luminescent phosphors.